I really doubt that the team could stuff in online play if they weren't planning on including it. Their entire engine and architecture would not be designed with online play in mind. At worst, they would need to rewrite significant portions of the code to allow decent online performance, and that is certainly a likely possibility. If they have to redo portions of their underlying engine, then they would need to reimplement the various components of the architecture that depend on those aspects of the engine. It could get ugly, messy, and time-consuming. Besides, the team will certainly already have their hands full in the last few months of development and they won't have the resources to devote to it without having to significantly delay the game (and past the important holiday season).

Let me use an example: Halo PC/Halo:Custom Edition vs Halo 2. HPC has poor netcode, while the netcode for H2 runs the game very smoothly. Bungie released Halo as an offline Xbox title. Later, Microsoft contracted Gearbox Software to port Halo to the PC, resulting in Halo PC (and later Halo: Custom Edition). Gearbox had serious issues with implementing online play when they were developing the title. Why? None of the architecture was designed for working over a high-latency (online, non-LAN) network. They had to rewrite a lot of code to get it to function adequately, but even in the final product, online play is still buggy.

Gearbox President Randy Pitchford made a huge post on their forums addressing all the criticisms of Halo PC and why it was how was.

Regarding why the netcode was not as great as people would like it to be:
"Halo PC netcode is what it is for a reason. It had to be the way it is and any experienced and capable networking programmer would've made similar conclusions."
"As we got to the end of the project, I pushed hard. I wanted that same feel as the synched/deterministic LAN game on the Xbox. The engineers sat me down and told me the score, drew diagrams about the software architecture, the way the internet works and the challenges that they faced with the specification and they showed me why it had to be that way. The specification dictated the code and vice versa..."

Regarding how much effort it took to implement good networking code in a game not designed fo it:
"...For the last several months, the Bungie and Microsoft guys were in it just as deep as the Gearbox guys were - writing code, prioritizing tasks, etc. It was a group effort and it was pretty hard-core, non-stop - even bonding, if you can call it that."

Brawl may not be simple enough to allow great code to be written:
"Okay - Now lets compare to, say, Counter-Strike.
Counter-Strike has only hit-scan weapons. There aren't big explosions that move things around (and when things aren't around, no one else needs to care about whether or not they are in the same place). When a Warthog moves, we MUST make sure that every client knows it's in the right spot. When a guy dies in Counter-Strike (even in CS Source, which has ragdoll), none of the clients care where the body falls. It's okay for it to be different for each client.
So, they have a game design that keeps the problem from showing itself."

On the difficulty of building on top of pre-existing code
"Halo's networking system was from scratch, first generation, developed in six months by three engineers and launched into a very competitive world of PC on-line gaming with a game design that is not condusive to the inherent problems of the internet. This networking system had to be meshed into an existing C code-base that had been evolved over 10 years of Bungie games until, in it's last usage, was completely overhauled, retuned and refitted with bubblegum, duct-tape and infinite sleepless work nights specifically for the Xbox platform (which was being invented while all of this refitting was going on)."

These are the real problems real developers face. I just don't think anything will change this late in the game, so we best hope it does have online, because the decision is already made.